Powders which comprise organosilicon compounds and are redispersible in water are known from DE-A-19535833. The powders are based on vinyl acetate copolymers and comprise on their surface not only dispersants but also, as hydrophobicizing agents, organosilicon compounds on water-insoluble support materials. This reduces the blocking tendency while maintaining the redispersibility of the redispersible powders. Blocking impairs the redispersibility and also the storage stability of the redispersible powders. When stored at high atmospheric humidity, such redispersible powders display a tendency toward caking, and do not hydrophobicize building compositions with which they have been mixed.
Powders which comprise organosilicon compounds and are not redispersible in water are known from GS-A-1217813. The powders are produced from lime and an emulsion of organopolysiloxane resin. The lime powders described there are used as hydrophobicizing agents for compositions comprising inorganic binders or consisting of such binders, and also as binders in mortars and renders.
The lime powders easily lead to caking and thickening of the compositions with which they have been mixed. In addition lime acts as a catalyst for reactive organosilicon compounds. Owing to their high alkalinity, the lime powders are only suitable for hydraulically setting compositions comprising lime and/or cement, or for gypsum plaster. The hydrophobicizing action is too low.